Implantable medical lead designs

ABSTRACT

The invention is directed to medical leads for use with implantable medical devices. Various features of medical leads are described, many of which may be useful in a variety of different leads, e.g., used in a variety of different applications. In one embodiment, the invention provides a medical lead of varying stiffness characteristics. In another embodiment, the invention provides a medical lead having a semi-conical shaped distal tip that becomes wider at more distal tip locations. In either case, described lead features may be particularly useful for J-shaped lead configurations used for implantation in a patient&#39;s right atrium. Many other types of leads, however, could also benefit from various aspects of the invention.

[0001] This application claims priority from U.S. ProvisionalApplication Serial No. 60/422,955, filed Oct. 31, 2002, the entirecontent of which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The invention relates to medical devices and, more particularly,to implantable medical leads for use with implantable medical devices(IMDs).

BACKGROUND OF THE INVENTION

[0003] In the medical field, implantable leads are used with a widevariety of medical devices. For example, implantable leads are commonlyused to form part of implantable cardiac pacemakers that providetherapeutic stimulation to the heart by delivering pacing, cardioversionor defibrillation pulses. The pulses can be delivered to the heart viaelectrodes disposed on the leads, e.g., typically near distal ends ofthe leads. In that case, the leads may position the electrodes withrespect to various cardiac locations so that the pacemaker can deliverpulses to the appropriate locations. Leads are also used for sensingpurposes, or both sensing and stimulation purposes.

[0004] In addition, implantable leads are used in neurological devicessuch as deep-brain stimulation devices, and spinal cord stimulationdevices. For example, leads may be stereotactically probed into thebrain to position electrodes for deep brain stimulation. Leads are alsoused with a wide variety of other medical devices including, forexample, devices that provide muscular stimulation therapy, devices thatsense chemical conditions in a patient's blood, gastric systemstimulators, implantable nerve stimulators, implantable lower colonstimulators, e.g., in graciloplasty applications, implantable drug orbeneficial agent dispensers or pumps, implantable cardiac signal loopsor other types of recorders or monitors, implantable gene therapydelivery devices, implantable incontinence prevention or monitoringdevices, implantable insulin pumps or monitoring devices, and the like.In short, medical leads may be used for sensing purposes, stimulationpurposes, drug delivery, and the like.

[0005] A number of challenges exist with respect to medical leads. Inparticular, new and improved lead designs are often needed to facilitatemedical implantation to specific locations within a patient. Forexample, the stiffness characteristics of a medical lead may affect theability to bend or conform a medical lead to a desired configuration. Astylet is often used to bend or form a distal region of the medical leadinto a configuration that can allow for implantation of the lead tipinto patient tissue at a desired location. As one example, J-shapedstylets may be inserted into a lumen of a medical lead to define aJ-shaped configuration of a distal region of the medical lead once thedistal region is inside a heart chamber. In that case, the distal tip ofthe lead may be implanted near the top of the right atrial chamber.Stiffness characteristics of the medical lead may affect the ability toachieve such a desired shape, however, and may also affect the shape ofthe medical lead following removal of the stylet.

[0006] Tissue fixation is another challenge relating to medical leads.In particular, a tip on the distal end of the medical lead may definecertain shapes to improve fixation to tissue, and possibly harness theeffects of fibrous tissue growth in order to anchor the lead tip in thetissue of a patient. For example, conventional leads commonly make useof distal tines to facilitate such anchoring in patient tissue. Distaltines, however, make lead removal much more traumatic to a patientbecause the tines can cause significant tissue damage upon removal fromtissue. Moreover, the ability to adequately anchor a lead tip in tissuecan also be complicated when the lead assumes different shapes, such asa J-shaped distal tip.

BRIEF SUMMARY OF THE INVENTION

[0007] The invention is directed to implantable medical leads for usewith implantable medical devices. Various features of medical leads aredescribed, many of which may be useful in a variety of different leadsused in a variety of different applications. As one example, thefeatures described herein may be particularly useful in leads designedfor implantation in a patient's right atrium. In that case, the lead canbe designed to facilitate formation of a J-shaped distal regionfollowing implantation of the lead in the patient's right atrium. AJ-shaped stylet may be inserted through a lumen of the medical lead toform the J-shaped distal region.

[0008] In one embodiment, the invention provides a medical lead ofvarying stiffness characteristics. The features that facilitate thevarying stiffness may be useful in a wide variety of applications,including specific applications in which the lead assumes a J-shapeddistal region for implantation in a patient's right atrium. In thatcase, the distal region of the implanted lead may benefit from enhancedstiffness in order to ensure that the distal region maintains theJ-shape following removal of a J-shaped stylet. In order to provideimproved stiffness at one or more lead locations, a medical lead maycomprise a first coiled portion including N filar(s), and a secondcoiled portion electrically coupled to the first coiled portion. Thesecond coiled portion may include N+M filars to define increasedstiffness of the second coiled portion relative to the first coiledportion, wherein N and M are positive integers.

[0009] In another embodiment, the invention provides a medical leadhaving semi-conical shaped distal tip that becomes wider at more distaltip locations. In other words, the distal tip tapers radially outward.Semi-conical distal tip features may find uses in a variety of leadapplications, including specific applications in which the lead assumesa J-shaped distal region for implantation in a patient's right atrium.The semi-conical shaped tip may provide a structure that allows fibroustissue growth to anchor the lead, but may be less aggressive thanconventional tines, allowing removal without substantial tissuemutilation. Moreover, the semi-conical shape may harness an inherentspring force of a J-shaped lead configuration such that an axial forcecomponent of forces that counterbalance the inherent spring force can beused to force the lead tip against tissue of a patient's atrium.

[0010] For example, a medical lead may comprise a lead body defining aproximal end for attachment to a medical device and a distal end forimplantation at a location in a patient. The medical lead may furthercomprise a semi-conical shaped tip at the distal end, the semi-conicalshaped tip becoming wider at locations further from the proximal end.

[0011] In other embodiments the invention may be directed to animplantable medical device (IMD) including a housing to house circuitry,and a medical lead electrically coupled to the circuitry. The medicallead may include the features mentioned above, such as first and secondcoiled portions to allow for variable stiffness of a first portionrelative to a second portion, or a semi-conical shaped distal tip toimprove fixation of the lead tip to tissue and possibly harness springforces in a useful way. In some cases, the lead may include both thefirst and second coiled portions to allow for variable stiffness, andthe semi-conical shaped distal tip to improve tissue fixation.

[0012] In still other embodiments, the invention may be directed to oneor more methods. For example, a method of creating a medical lead mayinclude coiling a first set of N filar(s) to define a first portion of amedical lead, and coiling a second set of N+M filars to define a secondportion of a medical lead having increased stiffness relative to thefirst portion, wherein N and M are positive integers. The method mayfurther include electrically coupling the first set of N filar(s) to thesecond set of N+M filars.

[0013] In another embodiment, a method may include inserting a J-shapedstylet into a lumen of a medical lead, implanting a semi-conical distaltip of the medical lead into tissue of a patient, and removing theJ-shaped stylet from the lumen.

[0014] The different embodiments may be capable of providing a number ofadvantages. For example, the use of a varying number of filars in a leadcoil at selected positions along the length of a medical lead canimprove stiffness characteristics of medical leads. Moreover, the use ofa varying number of filars in a lead coil can achieve improved stiffnesswith less impact on bending stress on the filars in the lead. In otherwords, varying the number of filars can be used to increase stiffnesswithout making bending stress to filars of the lead unacceptable forcertain applications. Such features may be particularly useful for leadsdesigned to assume a J-shape following implantation, but may beadvantageous in numerous other applications as well.

[0015] The semi-conical distal tip features can provide advantages interms of improved tissue fixation to the lead, e.g., by fibrous tissuegrowth around the tip, and may also be useful in harnessing springforces to force the lead tip against tissue. Moreover, a semi-conicaldistal tip may be removable from fibrous tissue with significantly lesstrauma to a patient than the removal of lead tips that include tines. Insome cases, the semi-conical distal tip may be designed such that theconical shape increases in thickness by no more than 25 percent, whichmay ensure that removal can be made without substantial tissuemutilation. Instead, the tissue may stretch, allowing removal of thelead with reduced trauma relative to lead tips that include tines.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a conceptual diagram illustrating an exemplaryimplantable medical device (IMD) in a human body.

[0017]FIG. 2 is a cross-sectional side view of an implantable medicallead according to an embodiment of the invention.

[0018]FIG. 3 is a top view of a coil structure within the medical leadillustrated in FIG. 2.

[0019]FIG. 4 is a cross-sectional side view of an exemplary electricallyconductive bus that may be used in a medical lead to couple N filar(s)to N+M filars.

[0020] FIGS. 5-7 are cross-sectional side views of medical leadsaccording to embodiments of the invention.

[0021]FIG. 8 is a top view of an embodiment of first and second coiledportions of a medical lead in which one filar is welded to another filarat the juncture of the first and second coiled portions.

[0022]FIG. 9 is an exemplary cross sectional side view of a distal endof a medical lead assuming a J-shaped configuration.

[0023]FIG. 10 is another exemplary cross sectional side view of a distalend of a medical lead assuming a J-shaped configuration.

[0024]FIG. 11 is a side view of a distal tip of a medical lead

[0025]FIG. 12 is a side view of a distal tip of exemplary medical leadincluding ridges to improve lead removal.

[0026]FIGS. 13 and 14 are cross-sectional front views of distal tips ofmedical leads including ridges to improve lead removal.

[0027]FIG. 15 is a side view of a J-shaped distal tip of a medical leadimplanted against tissue of a patient.

DETAILED DESCRIPTION OF THE INVENTION

[0028] The invention is directed to medical leads for use in implantablemedical devices. Various features of medical leads are described, manyof which may be useful in a variety of different leads used for avariety of different applications. In one embodiment, the inventionprovides a medical lead of varying stiffness characteristics. In anotherembodiment, the invention provides a medical lead having a semi-conicalshaped distal tip that becomes wider at more distal tip locations. Inother words, the distal tip tapers radially outward. The distal tip maybe semi-concial in that it takes a form that corresponds to a portion ofa cone. These and other embodiments described herein may be used toimprove medical leads for a wide variety of applications. Suchapplications may include specific applications in which a distal end ofthe lead is implanted in the roof of a patient's right atrium. Whenimplantation in the right atrium is desired, the lead may be formed intoa J-shape at a distal end of the lead, e.g., so that the lead tip can beimplanted in the roof of the patient's right atrium.

[0029]FIG. 1 is a conceptual diagram illustrating an exemplaryimplantable medical device (IMD) 10 in a human body 5. A similar devicemay also be used with other living beings. IMD 10 comprises a housing 12containing various circuitry that controls IMD operations. Housing 12 istypically hermetically sealed to protect the circuitry. Housing 12 mayalso house an electrochemical cell, e.g., a lithium battery for poweringthe circuitry, or other elements. The circuitry within housing 12 may becoupled to an antenna to transmit and receive information via wirelesstelemetry signals.

[0030] IMD 10 may comprise any of a wide variety of medical devices thatinclude one or more medical leads and circuitry coupled to the medicalleads. By way of example, IMD 10 may take the form of an implantablecardiac pacemaker that provides therapeutic stimulation to the heart.Alternatively, IMD 10 may take the form of an implantable cardioverter,an implantable defibrillator, or an implantable cardiacpacemaker-cardioverter-defibrillator (PCD). IMD 10 may deliver pacing,cardioversion or defibrillation pulses to a patient via electrodesdisposed on distal ends of one or more leads 2. In other words, one ormore leads 2 may position one or more electrodes with respect to variouscardiac locations so that IMD 10 can deliver pulses to the appropriatelocations.

[0031] The invention, however, is not limited for use in pacemakers,cardioverters of defibrillators. Other uses of the leads describedherein may include uses in patient monitoring devices, or devices thatintegrate monitoring and stimulation features. In those cases, the leadsmay include sensors disposed on distal ends of the respective lead forsensing patient conditions.

[0032] Also, the leads described herein may be used with a neurologicaldevice such as a deep-brain stimulation device or a spinal cordstimulation device. In those cases, the leads may be stereotacticallyprobed into the brain to position electrodes for deep brain stimulation,or into the spine for spinal stimulation. In other applications, theleads described herein may provide muscular stimulation therapy, gastricsystem stimulation, nerve stimulation, lower colon stimulation, drug orbeneficial agent dispensing, recording or monitoring, gene therapy, orthe like. In short, the leads described herein may find usefulapplications in a wide variety medical devices that implement leads andcircuitry coupled to the leads.

[0033] Referring again to FIG. 1, lead 2 assumes a J-shapedconfiguration. In particular, a distal portion 16 of lead 2 may assumethe J-shaped configuration. By way of example, the distal portion 16which assumes the J-shaped configuration may comprise approximately thedistal 80 millimeters of lead 2, although larger or smaller J-shapescould also be used.

[0034] In order to achieve a J-shaped distal portion 16, lead 2 mayfirst be implanted into the patient's right atrium. A J-shaped styletcan be straightened and inserted through a lumen of lead 2. Once adistal portion of the stylet is completely inserted into the lumen, thedistal portion of the stylet may assume the J-shape and thereby causethe distal portion 16 of lead 2 to likewise assume the J-shape. A distaltip 18 of lead 2, e.g., including an electrode, may then be implanted inthe roof of the patient's right atrium, such as between pectinatemuscles. As outlined in greater detail below, this distal tip 18 may beformed in a semi-conical shape in which distal tip 18 becomes thicker atmore distal locations. The distal tip may be semi-concial in that ittakes a form that corresponds to a portion of a cone. Such asemi-conical shape of distal tip 18 may improve fixation within thepatient, particularly when distal region 16 of lead 2 assumes theJ-shape for implantation in a patient's right atrial roof.

[0035] After implanting distal tip 18 in the right atrial roof, theJ-shaped stylet can be removed from the inner lumen of lead 2. Followingremoval of the J-shaped stylet, however, distal region 16 should stillretain the J-shape. Various features of lead 2 can help ensure thatinsertion and removal of the J-shaped stylet can result in distal region16 of lead 2 remaining in a J-shape. One such feature are filar coilsthat provide improved stiffness characteristics in distal region 16 tohelp ensure that lead 2 is flexible enough to assume the J-shape, butstiff enough to maintain the J-shape following removal of the stylet.Another such feature is a semi-conical shaped distal tip that canimprove fixation against tissue to help ensure that lead 2 does not loseits J-shape following removal of the stylet.

[0036] Lead stiffness is an important concern, particularly when thelead is designed to assume specific forms that facilitate implantationin specific locations within a patient. Again, the J-shapedconfiguration is only one example where stiffness is an issue. Manyother desired forms of a lead may also benefit from the stiffnessfeatures described herein.

[0037] Conventionally, increased stiffness, e.g., in a distal portion ofa lead, was achieved by increasing the pitch of a coiled filar thatelectrically couples the electrode on the distal tip of the lead to aproximal end of the lead. In particular, the filar could be coiled witha relatively small pitch to ensure flexibility in a major portion of thelead body. The term “pitch” refers to the longitudinal distance betweena first location of a filar and a second location of the same filarafter one coiled revolution about a lumen of the medical lead. Near thedistal portion of the lead, the pitch of the filar can be increased,which may increase the stiffness.

[0038] An increase in pitch of a filar, however, has several drawbacksparticularly in relation to filar stress when the lead is bent to agiven radius. For example, when the pitch of the filar increases, stressto the filar upon bending of the lead drastically increases. Morespecifically, bending of the lead in locations of increased filar pitchcould cause damage to the filar because the filar itself may physicallybend. For coils typically designed for this application, the coiledfilar stress it approximately proportional to the coil pitch for a givenbend radius. It is highly desirable to design a lead that can achieveincreased stiffness in one or more locations along a lead body, withoutcausing drastic stress increases to the filar(s) when the lead is bent.

[0039] In order to achieve improved lead stiffness without major adverseimpacts on mechanical filar stress, the invention may introduce variablenumbers of filars at different locations along a lead body. Morespecifically, a medical lead 2 may comprise a first coiled portionincluding N filar(s), and a second coiled portion electrically coupledto the first coiled portion. The second coiled portion may include N+Mfilars to produce increased stiffness in the second coiled portionrelative to the first coiled portion, wherein N and M are positiveintegers. The increased number of filars can improve stiffness of thelead at a desired location, such as in distal region 16 of lead 2. Theintroduction of additional filars can avoid drastic pitch increases inthe coils, however, ensuring that filar stress is more manageable. Thenumber of filars and the pitch of the filars in any given region of thelead may collectively define the lead stiffness in that region.Accordingly, these variables can be used to define a desired stiffnessfor various medical lead applications.

[0040] Other variables that can affect lead stress include the diametersof the filars and the diameters of the coils. Larger diameter filarsgenerally increases the lead stiffness and larger diameter coils of therespective filar generally decreases lead stiffness. These variables mayalso be defined so as to achieve a desired lead stiffness.

[0041]FIG. 2 is a cross-sectional side view of a medical lead accordingto an embodiment of the invention. FIG. 3 is a top view of a coilstructure in the medical lead illustrated in FIG. 2. Medical lead 22comprises a first coiled portion 24 including one coiled filar 25extending along a first segment of lead 22, and a second coiled portion26 including two coiled filars 27A, 27B extending along a second segmentof lead 22. An electrically conductive bus 28 electrically couples filar25 to filars 27A and 27B. In particular, electrically conductive bus 28may be an interconnect structure that provides both electrical andmechanical coupling of first and second portions. In first portion 24,the single filar 25 defines an electrically conductive path, and insecond portion 26, the two filars 27A, 27B define the electricallyconductive path. The introduction of additional filars in second potion26 causes the stiffness of second portion 26 to be larger than that offirst portion 24. Still, the stress in second portion 26, e.g., inresponse to bending, may be substantially reduced relative toconventional leads that achieve increased stiffness by increasing filarpitch rather than using an increased number of filars as describedherein.

[0042] The pitch refers to the longitudinal distance between a firstlocation of a filar and a second location of the same filar after onecoiled revolution about the lumen. As illustrated in FIG. 2, the pitchP₁ in first portion 24 is slightly smaller than the pitch P₂ in secondportion. The invention, however, is not limited in that respect, and inother configurations, the pitch P₂ can be made the same as or smallerthan the pitch P₁. In short, the introduction of additional filars candefine increased stiffness without regard to changes in pitch. Changesin pitch, however, can also affect stiffness. Thus, in accordance withthe invention, both the number of filars in any given portion of a lead,and the pitch of the filars in the given portion of the lead cancollectively define stiffness of the lead in the given portion of thelead.

[0043]FIG. 4 is a cross-sectional side view of an exemplary electricallyconductive bus 28 that may be used in a medical lead to couple Nfilar(s) to N+M filars. Electrically conductive bus 28 generallycomprises an electrically conductive material for coupling N filar(s) toN+M filars. For example, electrically conductive bus 28 may be acylindrical shaped structure with a through-hole 32 which forms part ofa lumen of the lead. The diameter of through hole 32 may be sized topermit a stylet to pass. Electrically conductive bus 28 may define afirst region 33 for electrically coupling to the N filar(s), and asecond region 34 for electrically coupling to the N+M filars.Preferably, electrically conductive bus 28 is formed of a biocompatiblemetal. Exemplary dimensions (in millimeters) of electrically conductivebus 28 are illustrated in FIG. 4, although a wide variety of differentshapes and sizes may also be used to achieve a bus in accordance withthe invention.

[0044]FIG. 5 is another cross-sectional side view of a medical lead 50according to an embodiment of the invention. In that case, first portioncoiled portion 54 includes one coiled filar 55, and second coiledportion 56 includes three coiled filars 57A, 57B, 57C. Electricallyconductive bus 58 electrically couples filar 55 to filars 57A-57C. Infirst portion 54, filar 55 defines an electrically conductive path, andin second portion 56, the three filars 57A, 57B, 57C define theelectrically conductive path. The introduction of a number of additionalfilars in second potion 56 causes the stiffness of second portion to belarger than that of first portion 54. However, the stress in secondportion 56, e.g., in response to bending, may be substantially reducedrelative to conventional lead stiffness features that use increasedpitch rather than an increased number of filars to achieve increasedlead stiffness.

[0045]FIG. 6 is another cross-sectional side view of a medical lead 60according to an embodiment of the invention. In lead 60, first portioncoiled portion 64 includes two coiled filars 65A, 65B, and second coiledportion 66 includes three coiled filars 67A, 67B, 67C. Electricallyconductive bus 68 electrically couples filars 65A and 65B to filars67A-67C.

[0046]FIG. 7 is another cross-sectional side view of a medical lead 70according to an embodiment of the invention. As shown in FIG. 7, medicallead 70 defines at least three coiled portions. A first portion coiledportion 74 includes one coiled filar 75, and second coiled portion 76includes two coiled filars 77A and 77B. Furthermore, a third coiledportion 78 includes three coiled filars 79A, 797B, 79C. Electricallyconductive bus 71 electrically couples filar 75 to filars 77A and 77B,and electrically conductive bus 72 electrically couples filars 77A and77B to filars 79A-79C.

[0047] Numerous other combinations of filars could also be used inaccordance with the invention In general, the invention provides amedical lead comprising a first coiled portion including N filar(s)extending along a first segment of the lead, and a second coiled portionelectrically coupled to the first coiled portion. The second coiledportion may include N+M filars extending along a second segment of thelead to define increased stiffness of the second coiled portion relativeto the first coiled portion, wherein N and M are positive integers. Insome cases, the portion defining increased stiffness may correspond to adistal end of the lead, and in other cases, the portion definingincreased stiffness may correspond to a proximal end of the lead. Instill other cases, the portion defining increased stiffness maycorrespond to a portion between the proximal and distal ends.

[0048] Also, varying levels of stiffness may be defined at any desiredlead location in accordance with the invention. For example, a firstportion may include N filar(s), a second portion may include N+M filars,a third portion may include N+M+O filars, a fourth portion may includeN+M+O+P filars, and so forth. N, M, O and P may represent positiveintegers. Alternatively a first portion may include N filar(s), a secondportion may include N+M filars, a third portion may include N+M−Ofilars. Put another way, a lead may include N+M+O filars, where N and Mare positive integers, and O is a positive or negative integer. Also, alead may include N+M+O+P filars, where N and M are positive integers,and O and P are positive or negative integers. A wide variety ofconfigurations of a lead may be defined in this manner in order toachieve desired stiffness for a given medical lead application.

[0049] As described above with reference to FIGS. 2-7, an electricallyconductive bus can be used to electrically couple the N filar(s) of oneportion of a medical lead to the N+M filars of another portion of amedical lead. To create such a lead, the filars can be wound around aninner core, and then the inner core can be removed. More specifically, acylindrical shaped electrically conductive bus may be inserted over aninner core, and N filar(s) can be wound around the inner core to definea first portion of a lead. The N filar(s) can be electrically coupled tothe electrically conductive bus on one side of the bus, and may bewelded, soldered, crimped or otherwise affixed to the bus to ensureelectrical contact. N+M filars can then be wound around the inner coreto define a second portion of the lead. The N+M filars can beelectrically coupled to the electrically conductive bus on the otherside of the bus, i.e., the side opposite the electrical contact to the Nfilar(s). The inner core can then be removed to define a lead having afirst coiled portion with N filar(s) and a second coiled portion withN+M filars. The location of the inner core can define a common lumenthat extends through the first coiled portion and the second coiledportion of the lead following removal of the inner core.

[0050]FIG. 8 is a top view of an embodiment of first and second coiledportions 81, 82 of a medical lead 80 in which one filar 84 is welded toanother filar 85 at the juncture of the first coiled portion 81 and thesecond coiled portion 82. In particular, a weld 87 may be applied toelectrically couple filar 84 to filar 85. In this manner, first andsecond coiled portions 81, 82 of a medical lead 80 can be defined inwhich first coiled portion 81 includes N filar(s) and second coiledportion 82 included N+M filars. The N+M filars of second coiled portion82 carry a common electrical potential, and are electrically coupled tothe N filar(s) of first coiled portion 81.

[0051] In order to create a medical lead as illustrated in FIG. 8, filar84 may be coiled around an inner core. Filar 85 may then be coiledaround a portion of the inner core. Filar 85 can then be welded to filar84 to define medical lead 80 that includes first coiled portion 81 andsecond coiled portion 82. The inner core can then be removed to define alumen inside the coiled portions 81, 82. In first portion 81, the singlefilar 84 defines an electrically conductive path, and in second portion82, the two filars 84 and 85 define the electrically conductive path.

[0052] Alternatively, filars 84 and 85 may be coiled together around aninner core. Filar 85 may then be cut, i.e., removed from first portion81. After cutting filar 85, filar 85 may be welded to filar 84 via weld87. The inner core can then be removed to define a lumen of lead 80.

[0053] The lead configuration illustrated in FIG. 8 may also define anynumber of filars. In general, first portion 81 may include N filar(s)and second portion 82 may include N+M filars, where N and M are positiveintegers. In the configuration of FIG. 8, the N filar(s) of firstportion 81 are the same filars as the N filar(s) of second portion 82.The M filar(s) of second portion 82 do not form part of first portion81.

[0054] The use of varying number of filars can also apply to bipolarleads or other types of multi-coil leads. A bipolar lead includes aninner coil and an outer coil. The inner coil is used to define anelectrical path for a first electrode, e.g., a ground electrode, and theouter coil is used to define a second electrode, e.g., a stimulationelectrode. Insulating tubing may be added around one or both coils.Varying number of filars may be used in a bipolar lead with respect toeither the inner coil, the outer coil, or both to define desiredstiffness characteristics.

[0055]FIG. 9 is a cross-sectional side view of a distal region of lead90 formed into a J-shape. Lead 90 may include an electrode 91 on adistal tip. A radio-opaque or echogenic ring 92 may be added as areference point for a physician so that a desired J-shape can beachieved. Accordingly, the location of ring 92 on lead 90 may be definedso that a J-shape of desired shape and radius can be more easilyachieved by a physician. Lead 90 may define two or more differentregions (labeled A, A₁, B, C and D). The different regions of lead 90may define different stiffness to help ensure that the J-shape can bemaintained following removal of a J-shaped stylet from an inner lumen oflead 90. An electrically conductive bus 94 can be used so that Nfilar(s) of regions A and A₁ can be electrically coupled to N+M filarsof regions B, C and D. Other variables of respective regions A, A₁, B, Cand D may also be selected to promote desired stiffness characteristics,including pitch, filar diameter, and the diameter of the coil(s).

[0056] TABLE 1, provided below, includes empirical evidence ofcharacteristics of a lead similar to that illustrated in FIG. 9. Thedifferent regions and number of filars per region are identified in thefirst column of TABLE 1. An electrically conductive bus was implementedto connect the two filars of region A₁ to the three filars of region B.For each region, the pitch, stress and bending stiffness are listed. Themeasured quantities were obtained from a bipolar lead in which the innercoil was substantially unchanged of the whole lead body. The outer coilincluded the measured variables of differing pitch and number of filarsper coiled region. TABLE 1 A A₁ B C D 2-FILAR 2-FILAR 3-FILAR 3-FILAR3-FILAR PITCH 0.57 0.57 0.78 0.9 1.15 (mm) STRESS 459 459 500 700 850(N/mm²) BEND 14.2 14.2 19.5 23.0 29.5 STIFFNESS (N * mm²/ radian)

[0057] TABLE 2 provides a reference for the data in TABLE 1. Themeasured quantities of TABLE 2 were obtained from a bipolar lead inwhich the inner coil was substantially unchanged of the whole lead body.The outer coil included the measured variables of differing pitch, butthe number of filars did not change in TABLE 2. The regions listed inTABLE 2 also correspond to the regions of lead 90 illustrated in FIG. 9,but the number of filars per region in TABLE 2 was held constant. TABLE2 A₁/A B C D 2-FILAR 2-FILAR 2-FILAR 2-FILAR PITCH 0.57 0.95 1.30 1.65(mm) STRESS 459 712 988 1282 (N/mm²) BEND STIFFNESS 14.2 18.9 23.4 30.1(N * mm²/radian)

[0058] Comparison of the data in TABLE 1 to that of TABLE 2 illustratesthe advantages that can be achieved by introduction of more filars toincrease stiffness. In particular, the data in TABLE 2 relative to thatof TABLE 1 illustrates that approximately the same bending stiffness canbe achieved with great reductions in stress when additional filars areintroduced. In particular, the data in TABLE 1 relative to TABLE 2achieved a 33% stress reduction.

[0059] TABLES 3 and 4 illustrate similar results. Again the data inTABLES 3 and 4 can be read with respect to J-shaped distal regions of aleads similar to lead 90 of FIG. 9. The measured quantities of TABLES 3and 4 were obtained from bipolar leads in which the inner coil wassubstantially unchanged of the whole lead body. The outer coil includedthe measured variables of differing pitch. The number of filars did notchange in TABLE 3, but did change in TABLE 4. With respect to TABLE 4,an electrically conductive bus was implemented to connect the filar ofregion A₁ to the two filars of region B. The filars of the differentleads quantified in TABLES 1-4 had 0.25 millimeter diameters, and thecoiled diameters were approximately 1.60 millimeters in every respectiveregion. In other words, the filar diameter and coiled diameter did notchange in the different leads quantified in TABLES 1-4. TABLE 3 A A₁ B CD 1-FILAR 1-FILAR 1-FILAR 1-FILAR 1-FILAR PITCH 0.57 0.57 0.90 1.30 1.70(mm) STRESS 473 473 767 1035 1320 (N/mm²) BEND 14.0 14.0 19.24 24.0730.2 STIFFNESS (N/mm²/ radian) COIL 1.6 1.6 1.6 1.6 1.6 DIAMETER (mm)

[0060] TABLE 4 A A₁ B C D 1-FILAR 1-FILAR 2-FILAR 2-FILAR 2-FILAR PITCH0.50 0.50 0.63 1.0 1.38 (mm) STRESS 406 406 509 800 1090 (N/mm²) BEND9.65 9.65 15.3 20.65 26.0 STIFFNESS (N/mm²/ radian) COIL 1.6 1.6 1.6 1.61.6 DIAMETER (mm)

[0061] Comparison of the data in TABLE 3 to that of TABLE 4 furtherillustrates the advantages that can be achieved by introduction of morefilars to increase stiffness. In particular, the data in TABLE 4relative to that of TABLE 3 illustrates that approximately the samebending stiffness can be achieved with great reductions in stress whenadditional filars are introduced.

[0062]FIG. 10 is another cross-sectional side view of a distal region oflead 100 formed into a J-shape. Lead 100 may include an electrode 101 ona distal tip 102. Moreover, distal tip 102 may define a semi-conicalshape in which distal tip 102 becomes thicker at more distal locations.In other words, distal tip 102 tapers radially outward. Additionaldetails of the advantages of a semi-conical shaped distal tip areprovided below with reference to FIGS. 11-15

[0063] A radio-opaque or echogenic detectable ring 103 may be added as areference point for a physician so that a desired J-shape can beachieve. Accordingly, the location of ring 103 on lead 100 may bedefined so that a J-shape of desired shape and radius can be more easilyachieved by a physician. Lead 100 may define a number of differentregions (labeled A, A₁, B, C, D and E). The different regions of lead 90may define different stiffness to help ensure that the J-shape can bemaintained following removal of a J-shaped stylet from an inner lumen oflead 100. One or more electrically conductive buses 104A-104C can beused so a number of filars of a respective regions can be electricallycoupled to a different number of filars of a different region. Othervariables of respective regions A, A₁, B, C and D may also be selectedto promote desired stiffness characteristics. These other variablesinclude pitch, filar diameter, and the diameter of the coil(s).

[0064] TABLES 5-7 below include additional empirical evidence ofcharacteristics of a lead similar to that illustrated in FIG. 10. Thedifferent regions and number of filars per region are identified in thefirst column of each of TABLES 5-7. For each region, the pitch, stress,bending stiffness and filar diameter are listed. Electrically conductivebuses were implemented to connect the filars of adjacent regions inwhich the number of filars changed. The measured quantities wereobtained from a bipolar lead in which the inner coil was substantiallyunchanged of the whole lead body. The outer coil included the measuredvariables of differing pitch and number of filars per coiled region. Thecoil diameter of the outer coil of the respective leads quantified inTABLES 5-7 was approximately 1.6 millimeters in every region. TABLE 51-FILAR 2-FILAR 2-FILAR 3-FILAR PITCH 0.50 0.65 1.0 0.9 (mm) STRESS 406525 796 722 (N/mm²) BEND 9.64 15.6 20.6 25.9 STIFFNESS (N/mm²/radian)FILAR 0.25 0.25 0.25 0.25 DIAMETER (mm)

[0065] TABLE 6 1-FILAR 2-FILAR 3-FILAR 3-FILAR (mm)H 0.50 0.65 0.86 0.9mm STRESS 406 525 693 722 (N/mm²) BEND 9.64 15.6 25.07 25.9 STIFFNESS(N/mm²/radian) FILAR 0.25 0.25 0.25 0.25 DIAMETER (mm)

[0066] TABLE 6 1-FILAR 2-FILAR 3-FILAR 4-FILAR PITCH 0.50 0.65 0.86 1.20(mm) STRESS 406 525 693 953 (N/mm²) BEND 9.64 15.6 25.1 41.1 STIFFNESS(N/mm²/radian) FILAR 0.25 0.25 0.25 0.25 DIAMETER (mm)

[0067] The data in TABLES 5-7 further illustrate the advantages that canbe achieved by introduction of more filars to increase stiffness. Inparticular, the use of additional filars to increase stiffness canachieve higher quantities of stiffness, and also reduced quantities ofbending stress. This is highly advantageous, particularly for medicalleads designed to assume shapes that facilitate implantation in hard toreach locations. The J-shaped lead is only one example.

[0068] Other variables that can affect lead stiffness include thediameter of the filars and the diameter of the coils. Larger diameterfilars generally increases stiffness and larger diameter coils of therespective filar generally decreases stiffness. These variables may alsobe defined so as to achieve a desired lead stiffness. For example, if afirst portion defines N filar(s) and a second portion defines N+Mfilars, one or more of the N+M filars of the second portion may havedifferent diameters than the N filar(s) of the first portion in order todefine a desired stiffness.

[0069] Also, the second portion may define a different coiled diameterthan the first portion, which could be accommodated by an electricallyconductive bus that tapers to change diameter at one end relative to theother end of the bus. In short, variables including the number offilars, the pitch of the filars, the diameter of the filars, and thediameter of the coils may be selected to promote a desired stiffness andfilar stress of a medical lead, and may change for different portions orregions of the lead in accordance with the invention.

[0070]FIG. 11 is a side view of a distal tip 111 of a medical lead 110.In particular, a semi-conical shaped tip 111 is formed on a distal endof lead 110. The semi-conical shaped tip 111 becomes wider at moredistal locations, i.e., tip 111 becomes larger at locations further froma proximal end of lead 110. In other words, the distal tip 111 tapersradially outward. An electrode 115 or other element such as a sensor maybe located on distal tip 111. The tip is referred to as semi-conicalbecause it takes a form that corresponds to a portion of a cone.

[0071] A semi-conical distal tip 111 may find uses in a variety of leadapplications, including specific applications in which lead 110 assumesa J-shaped distal region for implantation in a patient's right atrium.The semi-conical shaped tip 111 may provide a structure that allowsfibrous tissue growth to anchor lead 110, but may be less aggressivethan conventional tines, allowing removal without substantial tissuemutilation. In other words, semi-conical distal tip 111 can be removedfrom fibrous tissue with significantly less trauma to a patient than theremoval of lead tips that include tines.

[0072] Semi-conical distal tip 111 may be designed such that the conicalshape increases in thickness by no more than 25 percent. In other words,a radius R₂ may be less than approximately 125 percent of the radius R₁.Angle (α) as well as length (L) may be defined to ensure that radius R₂is larger than radius R₁ by between approximately 10 and 25 percent.Such sizes of radii R₁ and R₂ may ensure that removal can be madewithout substantial tissue mutilation. Instead, the tissue may stretch,allowing removal of the lead with reduced trauma relative to lead tipsthat include tines. Tissue stretching beyond 25 percent is veryunlikely, so the upper bound of radius R₂ being no greater than 25percent larger than R₁ can help ensure that tissue stretching canaccommodate removal of lead 110. Larger variations between R₁ and R₂,however may be useful as well.

[0073]FIG. 12 is a side view of a distal tip 121 of exemplary medicallead 122 including ridges 123 to improve lead removal. The outer FIGS.13 and 14 are cross-sectional front views of distal tips 121A, 121B ofmedical leads including ridges 123A-123C (FIG. 13) and 123D-123G (FIG.14) to improve lead removal. Medical lead 120 defines a semi-conicalshaped tip 121 formed on a distal end of lead 120, which can provide thesame advantages mentioned above in relation to FIG. 11. In addition, oneor more ridges 123 can further improve lead removal from tissue. Suchimproved lead removal can reduce patient trauma. An outer radius of theridges may be less than R₂ which can ensure that the ridges do not causeexcessive tissue stretching upon removal of the lead. Also the distanced may be less than half of length L.

[0074]FIG. 15 is a side view of a J-shaped distal region 151 of amedical lead 150 implanted against tissue 154 of a patient. Tissue 154,for example, may correspond to pectinate muscles of a patients rightatrial roof. Thus, distal tip 152 may be implanted between two pectinatemuscles. Lead 150 is substantially similar to lead 110 of FIG. 11 inthat distal tip 152 defines a semi-conical shape that becomes larger atmore distal regions. If desired, lead 150 may optionally include ridgesas illustrated in FIGS. 12-14.

[0075]FIG. 15 illustrates an additional advantage that can be achievedwith a semi-conical shaped distal tip 152 when used in a medical lead150 that defines a J-shaped distal region 151. As mentioned above, inorder to create the J-shaped distal region 151, a J-shaped stylet can bestraightened and inserted through a lumen of medical lead 150. Once adistal portion of the stylet is completely inserted into the lumen, thedistal portion of the stylet my assume the J-shape and thereby cause thedistal region 151 of medical lead 150 to likewise assume the J-shape.Distal tip 152 can then be implanted in tissue 155, which may correspondto the roof of the patient's right atrium. The stylet can then beremoved from the inner lumen of the medical lead.

[0076] Following removal of the stylet, the medical lead 150 may have anatural tendency to assume its original shape. In other words, thedistal region 151 may define a spring force 155 following removal of thestylet. Spring force 155 tends to force distal region 151 out of theJ-shape and into its original shape.

[0077] Semi-conical shaped distal tip 152 can harness spring force 155to improve anchoring in tissue 154. In particular, if distal tip 152 issemi-conical shaped having a larger radius at more distal locations, thenormal force (F_(NORMAL)) that counter balances spring force 155 willinclude an axial component (F_(AXIAL)) and a lateral component(F_(LATERAL)). In a static (non-moving) situation,

F_(LATERAL)=−(spring force 155), and

tan (α)=F _(AXIAL) /F _(LATERAL),

F_(AXIAL)=−F_(TIP)

F _(AXIAL)=tan (α)*F _(LATERAL), and

F _(AXIAL)=−tan (α)*(spring force 155)

[0078] Importantly, semi-conical shaped distal tip 152 can harnessspring force 155 to improve anchoring in tissue 154. The angle (α) canbe selected to define F_(AXIAL) so that enough anchoring force isachieved for any given use of medical lead 150. α may correspond toone-half of a cone angle of the semi-conical tip. The semi-conicalshaped distal tip 152 acts similar to a wedge when spring force 155 ispresent. Accordingly, semi-conical shaped distal tip 152 can be wedgedinto tissue 154 in response to spring force 154 to improve anchoring oftip 152 in tissue 154.

[0079] A number of embodiments of the invention have been described.However, one skilled in the art will appreciate that the invention canbe practiced with embodiments other than those disclosed. The disclosedembodiments are presented for purposes of illustration and notlimitation, and the invention is limited only by the claims that follow.

What is claimed is:
 1. A medical lead comprising: a first coiled portionincluding N filar(s) extending along a first segment of the lead; and asecond coiled portion electrically coupled to the first coiled portion,the second coiled portion including N+M filars extending along a secondsegment of the lead, the N+M filars producing increased stiffness of thesecond coiled portion relative to the first coiled portion, wherein Nand M are positive integers.
 2. The medical lead of claim 1, furthercomprising an electrically conductive bus to electrically couple the Nfilar(s) of the first coiled portion to the N+M filars of the secondcoiled portion.
 3. The medical lead of claim 1, further comprising aweld to electrically couple the M filar(s) of the second coiled portionto the N filar(s) of the first coiled portion, the N filar(s) of thesecond coiled portion being the same as the N filar(s) of the secondcoiled portion.
 4. The medical lead of claim 1, further comprising: aproximal end to be coupled to a medical device; and a distal end to beimplanted at a location in a patient, the second coiled portion beinglocated at the distal end.
 5. The medical lead of claim 1, furthercomprising a detectable indicator ring attached in proximity to thedistal end.
 6. The medical lead of claim 4, further comprising: a lumendefined by inner diameters of the first and second coiled portionsbetween the proximal and distal ends; and an insulating tubing coveringthe first and second coiled portions between the proximal and distalends.
 7. The medical lead of claim 6, further comprising an electrode onthe distal end and electrically coupled to the second coiled portion. 8.The medical lead of claim 6, the distal end having a stiffnesssufficient to maintain a J-shape following insertion and removal of aJ-shaped stylet through the lumen.
 9. The medical lead of claim 1,wherein a pitch of the first coiled portion is less than a pitch of thesecond coiled portion.
 10. The medical lead of claim 1, wherein a coildiameter of the first coiled portion is less than a coil diameter of thesecond coiled portion.
 11. The medical lead of claim 1, wherein one ormore of the N filar(s) of the first coiled portion have a filar diameterthat is different than a filar diameter of one or more of the N+M filarsof the second coiled portion.
 12. The medical lead of claim 1, furthercomprising a third coiled portion electrically coupled to the secondcoiled portion, the third coiled portion including N+M+O filarsextending along a third segment of the lead to produce differentstiffness in the third coiled portion relative to the second coiledportion, wherein O is an integer.
 13. The medical lead of claim 1,further comprising a fourth coiled portion electrically coupled to thethird coiled portion, the fourth coiled portion including N+M+O+P filarsextending along a fourth segment of the lead to produce differentstiffness in the fourth coiled portion relative to the third coiledportion, wherein P is an integer.
 14. The medical lead of claim 1,wherein the medical lead is a bipolar lead, wherein the first and secondcoiled portions collectively define an outer coil of a the bipolar lead,the medical lead further comprising an inner coil.
 15. The medical leadof claim 1, wherein N=1 and M=1.
 16. The medial lead of claim 1, whereinN=1 and M=2.
 17. The medial lead of claim 1, wherein N=2 and M=1.
 18. Animplantable medical device comprising: a housing to house circuitry; anda medical lead electrically coupled to the circuitry, the medical leadincluding: a first coiled portion including N filar(s); and a secondcoiled portion electrically coupled to the first coiled portion, thesecond coiled portion including N+M filars to produce increasedstiffness relative to the first coiled portion, wherein N and M arepositive integers.
 19. The implantable medical device of claim 18,further comprising an electrically conductive bus to electrically couplethe N filar(s) of the first coiled portion to the N+M filars of thesecond coiled portion.
 20. The implantable medical device claim 18,further comprising a weld to electrically couple the M filar(s) of thesecond coiled portion to the N filar(s) of the first coiled portion, theN filar(s) of the second coiled portion being the same as the N filar(s)of the second coiled portion.
 21. The implantable medical device ofclaim 18, further comprising: a proximal end to be coupled to a medicaldevice; and a distal end to be implanted at a location in a patient, thesecond coiled portion being located at the distal end.
 22. Theimplantable medical device of claim 21, further comprising a detectableindicator ring attached in proximity to the distal end of the medicallead.
 23. The implantable medical device of claim 21, furthercomprising: a lumen defined by inner diameters of the first and secondcoiled portions between the proximal and distal ends; and an insulatingtubing covering the first and second coiled portions between theproximal and distal ends.
 24. The implantable medical device of claim23, further comprising an electrode on the distal end and electricallycoupled to the second coiled portion.
 25. The implantable medical deviceof claim 23, the distal end defining a stiffness sufficient to maintaina J-shape following insertion and removal of a J-shaped stylet throughthe lumen.
 26. The implantable medical device of claim 18, wherein apitch of the first coiled portion is less than a pitch of the secondcoiled portion.
 27. The implantable medical device of claim 18, furthercomprising a third coiled portion electrically coupled to the secondcoiled portion, the third coiled portion including N+M+O filars todefine different stiffness relative to the second coiled portion,wherein O is an integer.
 28. The implantable medical device of claim 18,further comprising a fourth coiled portion electrically coupled to thethird coiled portion, the fourth coiled portion including N+M+O+P filarsto define different stiffness relative to the third coiled portion,wherein P is an integer.
 29. The implantable medical device of claim 18,wherein the medical lead is a bipolar lead, wherein the first and secondcoiled portions collectively define an outer coil of a the bipolar lead,the medical lead further comprising an inner coil.
 30. The implantablemedical device of claim 18, wherein the device is selected from thegroup consisting of: an implantable cardiac pacemaker, an implantabledefibrillator, an implantable cardioverter, an implantablepacemaker-defibrillator-cardioverter, an implantable sensing device; animplantable monitor; an implantable muscular stimulator; an implantablenerve stimulator; an implantable deep brain stimulator, an implantablegastric stimulator, an implantable colon stimulator, an implantableagent dispenser, and an implantable recorder.
 31. An apparatuscomprising: a first coiled portion including N filar(s); a second coiledportion including N+M filars to produce increased stiffness in thesecond coiled portion relative to the first coiled portion, wherein Nand M are positive integers; and means for electrically coupling thefirst coiled portion to the second coiled portion.
 32. The apparatus ofclaim 31, further comprising: a third coiled portion including N+M+Ofilars to produce different stiffness of the second coiled portionrelative to the first coiled portion, wherein O is an integer; and meansfor electrically coupling the second coiled portion to the third coiledportion.
 33. A method comprising: coiling a first set of N filar(s) todefine a first portion of a medical lead; coiling a second set of N+Mfilars to define a second portion of a medical lead having increasedstiffness relative to the first portion, wherein N and M are positiveintegers; and electrically coupling the first set of N filar(s) to thesecond set of N+M filars.
 34. The method of claim 33, whereinelectrically coupling the first set of N filar(s) to the second set ofN+M filars includes: coiling the first set of N filar(s) around a firstend of an electrically conductive bus; and coiling the second set of N+Mfilars around a second end of the electrically conductive bus.
 35. Themethod of claim 33, wherein electrically coupling the first set of Nfilar(s) to the second set of N+M filars includes welding the M filar(s)to the N filar(s), the N filar(s) of the first set being the same as theN filar(s) of the second set, wherein a weld defines a transitionbetween the first portion and the second portion.